1. Field of the Invention
This invention relates to a liquid crystalline compound and a liquid crystal composition. More particularly, it relates to a novel liquid crystalline compound containing a fluorine-substituted alkenyl group, a liquid crystal composition containing the same and a liquid crystal display element constituted by using the liquid crystal composition.
2. Description of the Related Art
Display elements using liquid crystalline compounds have been broadly utilized for watches, electronic calculators, word processors, etc. These display elements utilize optical anisotropy, dielectric anisotropy, etc.
Liquid crystal phases include nematic liquid crystal phase, smectic liquid crystal phase and cholesteric liquid crystal phase, but display elements utilizing nematic liquid crystal phase have been most broadly utilized. Further, the display modes include dynamic scattering (DS) mode, deformation of aligned phases (DAP) mode, guest-host (GH) mode, twisted nematic (TN) mode, supertwisted nematic (STN) mode, thin film transistor (TFT) mode, etc., but current mainstreams thereof are three modes of TN, STN and TFT. Among these, STN mode has been broadly used since it is collectively superior in the aspects of many characteristics such as display capacity, response speed, viewing angle, graduation property, etc., for liquid crystal display of simple matrix mode drive.
Liquid crystalline compounds used for thsese display modes should exhibit liquid crystal phases within a broad temperature range around room temperature, have a sufficient stability under conditions where the display elements are used and also have sufficient characteristics for driving the display elements, but at present, no single liquid crystalline compound satisfying these conditions has been found. Thus, it is the practical status that several kinds to several tens kinds of liquid crystalline compounds and if necessary, non-liquid crystalline compounds, have been blended to prepare liquid crystalline compounds furnished with required characteristics. These liquid crystal compositions are required to be stable to moisture, light, heat and air which are usually present under conditions where the display elements are used, and also stable to electric field or electromagnetic irradiation and moreover chemically stable to the blended compounds. Further, it is required for the liquid crystal compositions that values of various physical properties such as optical anisotropy value (.DELTA.n), dielectric anisotropy value (.DELTA..di-elect cons.), etc. are suitable, depending upon display mode and the shape of the display elements. Further, it is important for the respective components in the liquid crystal compositions to have good solubilities in each other.
Further, for the liquid crystalline compounds used for STN type display mode, it has been regarded as advantageous that the value of elastic constant ratio K.sub.33 /K.sub.11 (K.sub.33 : bend elastic constant and K.sub.11 : splay elastic constant) is large (see M. Akatuka et al: Proc. of Japan Display, 86 (1989), 388). When the elastic constant ratio K.sub.33 /K.sub.11 is made higher, change in the transmittance in the vicinity of threhsold voltage becomes steep so that a display element of high contrast is obtained. As compounds having a higher elastic constant ratio of K.sub.33 /K.sub.11, those having an alkenyl group on a side chain have been generally known (for example, see Japanese patent application laid-open No. Sho 61-83136, Japanese patent application laid-open No. Hei 1-151531, Japanese patent application laid-open No. Hei 1-157925, Japanese patent application No. Hei 2-104582, Japanese patent application laid-open No. Hei 3-223223, Japanese patent publication No. Hei 3-3643, Japanese patent application laid-open No. Hei 4-226929, Japanese patent application laid-open No. Hei 4-230352, Japanese patent publication No. Hei 4-30382, etc.). However, these compounds have a problem of being inferior in the solubility in liquid crystal compositions at low temperatures. Further, as compounds having a similar structure to that of the compound of the present invention, compounds disclosed in U.S. Pat. No. 5,183,587 have been known. However, these compounds have a problem of being far inferior in the heat stability. Thus, development of a liquid crystalline compound having a high elastic constant K.sub.33 /K.sub.11, a superior heat stability and an improved solubility at low temperatures has been long waited for.